Traditional optical imaging system design has focused on the combination of certain optics with an image detector. The optics may include, for example, lenses that focus and magnify images. The image detector may be a non-linear detector (e.g. film) or a linear detector (e.g., CCD array) that detects the optical radiation to form an image. The detected image may then undergo traditional image processing techniques—such as edge sharpening and color filter array (CFA) color interpolation—to improve certain aspects of the image. Thus, traditional optics in imaging systems are designed to form images at the detector that are sharp and clear over a range of field angles, illumination wavelengths, temperatures, and focus positions.
Traditional imaging systems, however, suffer from a number of setbacks. First, such systems typically have a relatively low depth of field, making it difficult to accurately and fully image real objects. The depth of field refers to the range of distances over which an optical imaging system can produce an in-focus image. Since the range of the depth of field is inherently limited, there are problems associated with imaging an object wherein its dimensions exceed the depth of field, as some portion of the image will be out of focus and blurry. To overcome this low depth of field, additional optical elements may be employed, for example an optical power absorbing apodizer or a stopped down aperture. However, overcoming low depth of field in these ways significantly reduces the optical energy at the image plane and decreases image resolution. The decrease of optical energy and increase in image noise may be at least partially remedied by adding significant illumination to the traditional imaging system; but this adds weight, complexity and expense to the design.
Misfocus-related aberrations, and control of such, present other problems in traditional imaging systems. Misfocus-related aberrations include, for example, chromatic aberration, curvature of field, spherical aberration, astigmatism, and temperature or pressure related misfocus; such aberrations cause image blurring and loss of resolution, and can make image colors appear shifted or otherwise incorrect. Traditional imaging systems remedy such aberrations by adding lens elements that increase cost, weight and overall lens complexity.
Consequently, traditional imaging systems must make a trade-off between (a) good images that generally require complex optical designs and (b) lower quality images that reduce size, weight, and/or the number of optical elements in the optical design.
Extended depth of field incoherent optical imaging systems (sometimes denoted as WAVEFRONT CODED® imaging systems) have been developed as an improvement over traditional imaging systems. These systems extend the depth of field (or depth of focus) properties in the imaging system by encoding the wavefront (“wavefront coding”) as it is processed through the imaging system. U.S. Pat. No. 5,748,371, issued to Cathey, Jr. et al., provides certain useful background on wavefront coding and is incorporated herein by reference.
One principle underlying wavefront coding is that an optical system, under incoherent illumination, forms a linear system with respect to intensity. As a result, wavefront coding is used with linear digital detectors (e.g., CCD array, CMOS arrays, etc.) since digital detectors can have a linear response to the intensity of optical radiation. Photographic film and many other detectors do not, however, demonstrate such a linear response. Even digital detectors, especially CMOS detectors, can be intentionally designed to exhibit a non-linear response. One reason to design such non-linearities is to extend the dynamic range of the detector; for example, the circuit around each pixel may be configured to produce a response that is logarithmic instead of linear. Another reason to design such non-linearities is to cause any portion of the image below a threshold of the detector not to be imaged.